test-bus-{vtable,introspect}: share data and test introspect_path()

test-bus-introspect is also applied to the tables from test-bus-vtable.c.

test-bus-vtable.c is also used as C++ sources to produce test-bus-vtable-cc,
and our hashmap headers are not C++ compatible. So let's do the introspection
part only in the C version.

src/test/meson.build

@@ -911,7 +911,8 @@ tests += [
          [],
          [threads]],
 
-        [['src/libsystemd/sd-bus/test-bus-vtable.c'],
+        [['src/libsystemd/sd-bus/test-bus-vtable.c',
+          'src/libsystemd/sd-bus/test-vtable-data.h'],
          [],
          []],
 
@@ -934,7 +935,8 @@ tests += [
          [threads],
          '', 'manual'],
 
-        [['src/libsystemd/sd-bus/test-bus-introspect.c'],
+        [['src/libsystemd/sd-bus/test-bus-introspect.c',
+          'src/libsystemd/sd-bus/test-vtable-data.h'],
          [],
          []],